In case that a polygon mirror has been used for a long time, the reflecting surfaces thereof become contaminated by a mist of lubricating oil scattered around from the ball bearing or the like of the polygon mirror driving apparatus, and this has been the cause for impairing the laser beam reflecting rate. To overcome this problem, one has contrived to prevent the mist of lubricating oil from scattering around the reflecting surfaces of the polygon mirror.
For example, a light deflecting system disclosed in Japanese Patent Publication No. Hei. 3-71690 shown in FIG. 4 is characterized as follows. A zigzag labyrinth is formed in order to prevent the scattering around of a mist of lubricating oil from a ball bearing 22 that fixes a vertical shaft 21 while interposed between an upper case 20 and the vertical shaft 21. The zigzag labyrinth is formed not only by fixing a shaped plate 24 on the upper end of a polygon mirror 23, the shaped plate 24 having projected portions so as to be zigzag in section, but also by securing another shaped plate 25 to the inner surface of the upper case 20 so that the shaped plate 24 is fitted with the shaped plate 25, the shaped plate 25 having recessed portions so as to be zigzag in section. Namely, a gap between the shaped plates 24, 25 corresponds to the labyrinth. For the same purpose, another labyrinth is formed by arranging a spacer 26 on the outer circumference of the vertical shaft 21 as high as the upper end of the polygon mirror 23 and further arranging an annular projected portion 27, so that the labyrinth can be formed utilizing a space between the spacer 26 and the annular projected portion 27.
This arrangement entails much time in preparing the complicatedly zigzag-shaped plates 24, 25, makes the assembling operation cumbersome due to a large number of parts being involved, and hence makes the light deflecting system itself costly and reduces productivity. It is for these reasons that the light deflecting system has been considered less user-friendly.
Further, since the zigzag-shaped plates 24, 25 are interposed between the upper end of the polygon mirror 23 and the upper case 20, the length of the vertical shaft 21 of the light deflecting system in the vertical direction, i.e., the height of the light deflecting system becomes large, which in turn has made it difficult to downsize and thin-size the light deflecting system.